To meet production schedules, the first 67 Navy airplanes, which would desirably have been equipped and designated as E-models, were equipped with the P&W TF-30P-408 engine.  Because of the engine difference and the inclusion of other components also used in the A-7B, which made these 67 somewhat of a hybrid model, the Navy decided to redesignate this lot as the A-7C.  Other B-model systems and features used in the C-model included the dual hydraulic system instead of the triple system, the ESCAPAC IG ejection seat versus the SJU-8/A, resulting in a minus 1542-pound empty weight and a small difference in performance.  Later, the Navy retrofitted these airplanes with the TF-41 engine, but retained  the C model designation (surely this was a move devised to confuse historians).

A-7:

Genesis of the Navy Attack Bomber, Light (VA(L))
In Comat
Farewell Proud Bird
Aircraft Performance Perspective